1. Field of the Invention
The present invention relates to an integrated photoscreen for making halftone reproductions from photographs, and a method for using the photoscreen to make such halftone reproductions, suitable for offset printing.
2. Description of the Prior Art
Heretofore much difficulty has been encountered in making a printing plate from a photograph utilizing conventional electrostatic platemakers. The difficulty has been primarily in the reproduction of gradations in color or lightness and darkness. In this respect, without using some sort of screen technique, the white areas or tones and the black areas or tones will come out on the plate with continuous white or dark areas rather than with a gradation of white or dark areas.
One previously proposed solution to the problem of creating a halftone printing plate was to create a printable dot contact screen sold under the trademark KODAK PMT by the Eastman Kodak Company. With the PMT.TM. screen, good reproduction is obtained with the halftones custom made to the capability of the printing press. In other words, the tone range, from highlight to shadow, of most original pictures is usually greater than the printing press is capable of printing. According to the KODAK.TM. system, good reproductions are made by measuring the press's capability to record or print highlight and shadow dots and then, working back from the press capability, to determine dot sizes required in the halftone to be created for the camera. The cameras utilized in electrostatic platemaking are manufactured by a number of companies, such as, for example, the 3M Company which manufactures the 2M 1165 Automatic Exposure Platemaker, the A. B. Dick Company which manufactures the A. B. Dick 145 Photomat.TM. Platemaker and the A. B. Dick 155 Electrostatic Master Maker, and by Madax Graphics Products, Incorporated, which makes the Madax VFV 215 Variable Focus Verilith System.
With the Kodak.TM. PMT system, a number of steps are required to produce good quality photoreproduction by electrostatic cameras. First a Kodak reflection density guide Q-16 is mounted on the copy board as you would mount a piece of copy.
Second, a sheet of Kodak PMT negative paper AD is placed on the camera back and covered with a Kodak PMT gray contact screen. This is then exposed using a main exposure only.
Third, the screened image is transferred to the Kodak PMT receiver paper in a special diffusion-transfer processor containing Kodak PMT activator.
Fourth, the resulting screen print should have just enough exposure to burn out the first two steps on the Kodak reflection density guide and to pick up dots in the third step. The 2.00 step in the Kodak reflection density guide should be completely black at this point.
In the fifth step, the screened print is mounted on the copy board and exposed at the same size to a sheet of Kodak PMT paper plate negative material using normal exposure.
In the sixth step, the PMT negative is processed with a Kodak PMT printing plate in a diffusion-transfer processor containing Kodak PMT activator.
Now the operator, after taking care of the make-ready, is ready to run a press sheet from the plate on the stock the operator will be using for the printing job.
Next, in the eigth step, the operator examines the press sheet and picks out the smallest printable dot from any density step and the largest printable dot from any density step and records the density steps producing these dots.
Finally, the original screen print is compared to the printed sheet to find the step on the screen print in which the smallest printable dot appeared on the printed sheet. The dot appearing in that step on the original screen print should then become the operator's "aim highlight" dot for all screen prints made using the PMT product.
Also from a comparison of the original screen print to the printed sheet, the operator will find the step on the screen print in which the largest printable dot appeared on the printed sheet and the dot appearing in this step on the screen print then becomes the "aim shadow" dot for all the screen prints made using the PMT product for that printing job.
As will be described in greater detail hereinafter, the integrated photoscreen of the present invention provides a simple means for making a good quality halftone printed reproduction on a printing plate of a photograph without the lengthy and cumbersome process of creating a specialized PMT.TM. product.
Also heretofore a number of proposals have been made for improving the making of a halftone reproduction, some of which proposals include the manufacture and use of a film mask. Examples of previously proposed systems utilizing film masks and/or describing various forms of film masks are disclosed in the following U.S. patents:
______________________________________ U.S. Pat. No. PATENTEE ______________________________________ 3,152,528 Pendry 3,905,822 Marks 3,192,510 Marks 3,961,847 Pendry 4,003,649 Goren et al 4,012,137 Goren 4,027,962 Mailloux 4,080,055 Gary 4,083,632 Mailloux et al 4,090,786 Bobbe 4,143,967 Wicker 4,227,795 Bobbe et al ______________________________________
With reference to the above noted patents and other patents various techniques have been proposed for improving the making of a halftone reproduction using a Xerographic or Electrostatic Process.
Large solid area development was the primary problem associated with Xerography, but the Xerographic and the Electrostatic processes were producing excellent results with text type and simple line art drawings. The idea to produce an electrostatic offset printing plate was a tremendous advance in producing good quality printing of type and line art drawing. The reproduction of a continuous tone picture or transparency with Xerographic or Electrostatic process, which is a very high contract process, cannot distinguish the varying densities of a continuous tone original document accurately. Large areas of solid image was the primary problem of Xerography.
One solution to the problem of the large solid areas of the Xerographic photoreceptor has been the adoption of development techniques other than the open cascade development system which alters the charge pattern present on large areas of contiguous charge on the photoreceptor. Many other techniques are well known, such as a magnetic brush, powder cloud or liquid development system, or by the use of development electrodes, for example, as disclosed in the Gundlack U.S. Pat. No. 2,777,418 or the Clark et al U.S. Pat. No. 2,952,241.
Other techniques include breaking up the contiguous charge on the photoreceptor by the use of mechanical, optical or electrocal techniques. For example, the Carlson U.S. Pat. No. 2,599,542 suggests using an electrophotographic plate which has been etched to resemble a waffle (grid design) and the Weigl U.S. Pat. No. 3,248,216 teaches discharging the electrostatic plate selectively to produce a pattern to break up a solid area.
Optical techniques for improving solid area coverage have been employed to discharge the solid area using a screened light source. The screen may consist of or take the form of a line or comb screen or a grid or dot pattern. The electrostatic plate is selectively discharged in those areas where light passes through the screen but retains its charge in those areas blocked by the opaque areas in the screen. Examples of optical techniques for improving solid area coverage may be found in U.S. Pat. Nos. 2,598,732; 3,121,010; 3,212,888; 3,335,003; and 3,535,036. These previously proposed techniques were, in effect, related to the Xerographic copying system and not particularly related to the art of producing a halftone reproduction of a continuous tone original document by a screen technique.
In the Pendry U.S. Pat. No. 3,152,528 there is disclosed a shadow-casing member comprising an array of opaque lines or opaque dot elements which break up the light into a halftone pattern. The resulting electrostatic images, after exposure, are subdivided into a halftone pattern, resulting in a configuration of the electrostatic fields above the photoconductive surface which causes the uniform halftone development of large solid areas and the rendering of continuous tones as halftones.
Accordingly, this patent teaches the use of a screen consisting of alternating opaque and transparent areas superimposed over a document to be copied between the document and the lens system of a Xeroxgraphic copy machine. This technique did improve the solid areas of the Xerographic process in producing an inaccurate copy of a continuous tone original.
Another approach has been the use of compound document screens as described in the Marks U.S. Pat. Nos. 3,905,822 and 3,912,510. More specifically, Marks teaches the use of a compound screen to extend the range of input densities transmitted or reflected from an original document over which there is a change of output density in a copy made using a high contrast electrophotographic process such as Xerography.
An object of the Marks technique is to extend the range of the Xerographic process by employing a compound document screen, consisting of a clear transparent substrate material, having on at least one surface thereof, a plurality of substantially opaque dots of uniform density, which screen is adapted to be positioned proximate to, preferably in contact with, the face of the document to be copied, between the document face and a lens system of an electrophotographic copying system. The dots in this screen consist of a plurality of substantially light-absorbing dots and a plurality of substantially light-reflecting dots.
The frequency and array of these dots is such that light which is reflected by the screen is modulated by the lens in accordance with the modulation transfer function of the lens system employed in a Xerographic copier.
Thus, the halftone screen consisted of a plurality of mixed, substantially opaque dots of uniform density, half of which are substantially equal light-absorbing dots and half of which are substantially equal light-reflecting dots. The substantially light-absorbing dots, referred to as black dots, should be of such density to absorb at least eighty percent of the light. Also, the light-reflecting dots, referred to as white dots, should reflect at least eighty percent of the light and optimum results are achieved as both values approach one hundred percent.
However, it is the lens system which modulates the light transmitted or reflected from the original document in accordance with the Modulation Transfer Function (MTF) of the particular lens system that is employed in an electrostatic copying machine. For a given lens system MTF, the frequency of the dot pattern is too low if the dot pattern is accurately imaged by a properly focused lens. In this case, the aerial image of the dot pattern would be a square wave which according to conventional Fourier Analysis, comprises sine waves at the fundamental dot pattern frequency and many higher harmonics. Such a square wave aerial image produces a single dot size on the photosensitive member.
The lens system employed in common Xerographic equipment, exhibits a rather large MTF as compared to the present high quality lens systems employed in electrostatic offset plate-making machines. As a result, the screen disclosed in the Marks patent will not give the desired result in halftone reproduction on a printing plate for continuous tone originals using electrostatic offset plate making machines. In this respect, the compound screen exhibits only substantially uniform dot sizes which will not change in size due to the effect of image transmittance or reflectance through the compound screen.
In contradistinction, the integrated photoscreen of the present invention produces varying amounts of light reflected or transmitted by the use of varying density, vignetting dots, which vary in density from a lightest density on the outside perimeters of the dots to the darkest core in the extreme center of the dot and by the use of light modulating white dots which, in effect, are modulated through the transparent substrate from the surface of the original document's continuous tones.
The integrated photoscreen of the present invention modulates the light reflected or transmitted by varying the density of the original document's continuous tone, or to reduce or decrease the density range of the original document's continuous tone to make a halftone reproduction which is suitable for the offset printing process and which is used as an electrostatic printing plate.
None of the prior art patents referred to above, disclose or suggest a film mask for reducing or decreasing the original document's continuous tone density range with such a film mask having an array of varying density, light modulating, vignetting, light gray dots, mixed with an array of light modulating white dots on a sheet of transparent film material.
In the Turner et al U.S. Pat. No. 3,961,847 there is disclosed an electrophotographic printing machine having an arcuate photoconductive member. An arcuate screen member is mounted in the printing machine and is closely spaced to the photoconductive member so as to produce half tone images of an original document. The screen member itself includes a plurality of lines or dots printed on a substantially transparent sheet.
In the Goren et al U.S. Pat. No. 4,003,649 there is disclosed a phase screen having a substrate made of a non-absorbing transparent material such as glass or plastic. A plurality of square cut rulings or grooves of a predetermined screen frequency extend in straight lines across the substrate. The phase screen is positioned above the surface such that the rulings are aligned parallel to the direction of rotation and orthogonal to the axis of the drum.
In the Goren et al U.S. Pat. No. 4,012,137 there is disclosed a screen member that continuously rotates and comprises a square array of dots thereon. Single color light images, namely, red, blue and green, pass through the screen member and irradiate a charged photoconductive member which dissipates the charge thereon to produce single color electrostatic latent images thereon.
In the Mailloux U.S. Pat. No. 3,027,962 there is disclosed an electrographic printing machine having a dot screen and a transparent platen with an opaque sheet interposed therebetween. A color filtered light image of a color transparency is formed on the platen whereby the dot screen modulates the color transparency image forming a half tone light image which is combined with the image of a composition frame forming a combined image. In this manner, a combined image is formed on the transparent platen, thus enabling a scanning system to form a flowing half tone light image thereof, and as such, a combined flowing light image which passes through the dot screen and composition frame.
In the Gary U.S. Pat. No. 4,080,055 there is disclosed a screen member including a transparent disc having a plurality of dots disposed thereon. The screen is positioned at discrete preselected angles through which successive color filtered light images pass. An unfiltered light image is transmitted through the screen with the screen being positioned at one of the preselected angles for the filtered light image.
In the Mailloux et al U.S. Pat. No. 4,038,632 there is disclosed a half-tone screen including a clear transparent substrate having a plurality of spaced opaque regions thereon. The opaque regions are arranged in at least a high frequency repetitive pattern and a low frequency repetitive pattern. The screen can be used to modulate the light image of a color transparency being reproduced by an electrophotographic printing machine.
In the Bobbe U.S. Pat. No. 4,090,786 there is disclosed a screen member including a transparent sheet having a plurality of hard dots thereon. A plurality of sets of dots are disposed on the transparent sheet wherein each set acts as a light filter permitting light rays having a discrete spectal band width to pass therethrough. Each set of light filtering regions is substantially complementary in color to the color of the filtered light image, and rotated through a pre-selected angle relative to the next adjacent set of light filtering regions.
In the Wicker U.S. Pat. No. 4,143,967 there is disclosed a dark screen having small openings which have been aligned at a specific angle in relationship to a screen member. The screen has two punched holes which represent the axis of the screen and which are used to register the screen with a film positioned beneath a halftone. As such, this patent discloses a method of producing a latent photograph by using a reproduction of the photograph in both negative and positive form, and photographing both negative and positive in sequence and in register through the dark screen.
In the Bobbe et al U.S. Pat. No. 4,227,796 there is disclosed an optical system in which a movable photosensitive member is exposed to a light image of an original document, including a light source which illuminates a screen having at least two adjacent rows of opaque dots. The screen is disposed in an opaque frame or slit and has a plurality of dots disposed thereon arranged in successive rows. In this manner, a finely divided charge pattern is recorded on the photosensitive member.
None of the prior art patents referred to above disclose or suggest a film mask to reduce or decrease an original document's (photograph's) continuous tone density range with a film mask having an array of varying density, light modulating, vignetting, light gray dots mixed with an array of light modulating white dots on a sheet of transparent film material as provided by the integrated photoscreen of the present invention as will be described in greater detail in the following description.